Newbie Question: New to iMac, need advice for memory upgrade

I just bought an iMac this week and would like to up grade the memory. I was going to max it out at 4GB, but when i start looking online...the price differences between the Apple Store and other sources stopped me. So I figure I'd better ask for some advice before I start buying.
I'm using an iMac 2.0 Core 2 Duo; 20"display/250GB drive, with the Anodized aluminum body (if that matters). Currently has the stock 1GB.
The Apple store wanted $1K for 4GB...and others were A LOT less.
Is it just a matter of buying 2x2GB 667MHz DDR2 (PC2-5300) or is there something else I need to be looking for, can I buy 2 2GB from:

Sorry, if the question is so basic I'm new to Mac's, well it's been many years since I've used one. I'm so used to working w/ PC's it's got me second guessing everything..except not wanting to continue using PC's.
Thanks!

I bought a 4Gb upgrade kit from Other World Computing (the link that Eidorian provided) about a month ago, for our new iMac. I think at the time it was around $125, looks like it's even less now. But yes, this is legit, and yes, it's unreal how much more Apple charges for its RAM.

I'm looking into doing the 4GB upgrade as well. Does anyone know anything about which brand is best? I'm looking at Patriot, Kingston, Mushkin and OWC. Any input would be very much appreciated. Thanks!

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